How can coral reefs deal with climate change? Get better roommates

Time is running out for coral reefs as the climate gets hotter. So scientists are searching the globe for corals that are better at enduring heat.

Now, new research shows how those "super corals" can survive: less roommate drama.

Reefs depend on a crucial partnership between the corals and the algae that live in the corals' tissue. The algae make food for the coral using sunlight and in exchange, get a nice spot to live.

But when oceans heat up, that relationship goes bad, and the corals kick the algae out. Without their roomies, corals can die, turning a ghostly white, bleached color.

Still, some corals seem to resist bleaching better than others. A new study shows that those corals depend on algae that are better at tolerating heat.

Researchers hope that pinpointing these abilities will help develop new conservation tools to preserve the world's reefs as temperatures rise. About a quarter of all marine life rely on coral reefs in some way, along with half a billion people around the world also depend on reefs for their food and livelihoods.

"Heat stress can kill a lot of corals really fast," says Kate Quigley, a research scientist at James Cook University and the Minderoo Foundation in Australia. "I hope that nature does have some mechanisms to get us through the next few years while we get our act together."

Heat waves are becoming more common in the ocean

Stretching more than 1,000 miles, Australia's Great Barrier Reef was once thought to be too big to fail. But in the last seven years, the reef has been hit with repeated underwater heat waves, causing four mass bleaching events.

When water temperatures rise, the tiny algae inside the corals begin producing toxins. The corals, also under stress from the heat, begin expelling the algae. In the process of evicting their tenants, the corals lose a key food source.

It's not an immediate death sentence for coral reefs, however. If the temperature decreases, corals can bounce back. But as marine heat waves become more common, that recovery gets tougher and tougher to achieve.

Still, even on the Great Barrier Reef, small pockets of coral have been able to withstand the heat waves that have hit in recent years.

"These reefs were just really cooking but for some reason, they were resilient," Quigley says.

Some corals have roommates that can take the heat

Quigley and her colleagues looked at samples from these reefs, along with others, and identified some of their key strategies in a new study in the journal Science Advances.

For one, the surviving corals housed strains of algae that could tolerate more heat. They were also able to build up their communities of algae quickly after the heat wave, essentially recruiting new tenants the fastest.

"It really helps to identify reefs that we really need to make sure that we're protecting," Quigley says. "Those resilient reefs, those reefs that are naturally good at surviving, we need to make sure we're releasing them of other pressures."

Quigley also found that heat-resistant algae were abundant on the reef after a marine heat wave, but unlike the corals that survived better, others corals couldn't take advantage.

"It would be amazing if corals could just switch," Patrick Buerger, research fellow in coral reef resilience at Macquarie University in Sydney, Australia who was not involved in the research. "Unfortunately, the switching is not as common as we were all hoping for."

Super corals can only survive so long

In addition to protecting the reefs that are home to these "super corals", some researchers hope that those corals could also be used to restore reefs that have been damaged by climate change and other impacts. But there may not be many to pick from.

"Even if we could engineer some super-duper coral, there might be just a few species that would live in that future world," says Lupita Ruiz-Jones, an assistant professor at Chaminade University of Honolulu who also was not part of the research. "And that's I think the really sad part for me — just imagining this world where we just have much less beauty in the water."

With the recent discoveries, researchers are now also focusing on the algae as a key part of helping reefs survive. In Australia, a team of researchers has been exposing algae to high temperatures, in an effort to develop a strain that's super-tolerant of heat.

While the hope is that those algae could also aid in reef conservation in the future, researcher Patrick Buerger says it's likely that it wouldn't help all coral species. And even the toughest corals can only endure so much. Currently, the world is on track for just under 3 degrees Celsius of warming by 2100, a level that would wipe out nearly all coral reefs.

"The action has to be on climate change," he says. "This is a short-term solution that might buy some time for corals to adapt. But the main focus has to be on climate. There's not a silver bullet to the problem."

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